The present invention relates to a fluorescent lamp used as a signal light, and more particularly, to a single-ended subminiature fluorescent lamp in which power supply terminals are arranged on only one end of a bulb. The bulb is shaped as a typical halogen lamp used as a signal light.
FIG. 4 illustrates a cross-sectional view along the lamp axis Z of a conventional fluorescent lamp 90. FIG. 5 provides another cross-sectional view of the lamp along a surface including the B--B line of FIG. 4. The conventional fluorescent lamp 90 comprises a bulb 94 with fluorescent material 94a coated on an internal surface of the bulb 94, a stem 91, a discharge chamber 94b filled with gas and mercury, lead wires 91a, 91a', 91a" passing air-tightly through the stem 91, a thermal cathode filament 92 coated with electron emitting material 92a and supported by the lead wires 91a and 91a', and a ring-like anode 93 supported by the lead wire 91a". To illuminate the conventional fluorescent lamp 90, a DC voltage of 5 V is first applied between the lead wire 91a and the lead wire 91a" such that thermal electrons are emitted. A DC voltage of 24V is then applied between the thermal cathode filament 92 and the ring-like anode 93, which directs the thermal electrons emitted from the thermal cathode filament 92 to the ring-like anode 93 such that discharge starts, thereby exciting fluorescent material 94a to emit light.
The conventional fluorescent lamp 90 has several problems. First, converting efficiency from wattage to luminance of the conventional fluorescent lamp 90 is approximately 3.7 lm/W, which is enough to use as a signal light, but insufficient for use as a backlight of a liquid crystal display. Second, as shown in FIG. 6, although the discharge should occur between the ring-like anode 93 and a grounded end S of the thermal cathode filament 92, a discharge spot P on the grounded end S moves towards a thermal cathode end U on a positive side as time passes, due to deterioration of the electron emitting material 92a. When a DC voltage of 5 V is applied to the thermal cathode filament 92, the discharge spot P on the thermal cathode filament 92 moves to the center point T of the thermal cathode filament 92, which has a higher voltage by approximately 2.5V than at the grounded cathode end S. In other words, the electric potential between the thermal cathode filament 92 and the ring-like anode 93 decreases by approximately 2.5 V, thereby decreasing discharge current and the luminance of the subminiature fluorescent lamp 90. When the discharge spot P passes the center point T of the thermal cathode filament 92 toward the thermal cathode end U on the positive side, the luminance of the fluorescent lamp 90 further decreases, as shown in line BO of FIG. 3. This is because the discharge distance between the discharge spot P and the ring-like anode 93 increases in spite of the voltage decrease between the thermal cathode filament 92 and the ring-like anode 93.